AVIATION experts believe a “cat­astrophic” mechanical failure left police helicopter pilot David Traill with no option but to try to crash land on the pub roof.

His Eurocopter EC135 T2 is used by most of our police forces and air ambulance services and has a good safety record.

The craft, which can carry seven passengers and reach speeds of up to 157mph, is popular with pilots because if it loses one of its twin engines it can still fly.

Even without both engines it will glide and can be safely landed, unless severe mechanical failure leaves the pilot with little or no control, as is ­suspected in this case. A Eurocopter EC135 T2 crashed in Essex in 2007 because its autotrim system, which maintains the craft’s position, failed and pitched it nose-down.

The Glasgow crash inquiry will examine whether this system deactivated after the engines failed because one witness said the helicopter fell “like a stone” without its rotor blades turning.

Struan Johnston, director of Caledonian Aviation, said: “We know from the eyewitness accounts the aircraft landed on the roof almost vertically.

“The rotor blades were not turning or were turning very slowly, which indicates some kind of catastrophic failure in the drive train or the engine system. The pilot would have done everything in his power to try to land the aircraft safely. He nearly pulled it off but the weight of the aircraft just would not have been sustainable on the roof.”

A helicopter similar to the one involved in the horror crash [PA]

The pilot would have done everything in his power to try to land the aircraft safely

Struan Johnston, director of Caledonian Aviation

He added: “Operating in an urban environment has many hazards but most pilots are trained by the military and they are used to flying in these ­conditions.

“People have asked why the pilot in this case did not try to land in the road nearby but a propeller coming into contact with the edge of a building or power lines is not good. I think he selected the best of a lot of bad options by trying to land on the roof of the pub.”

David Learmount, of aviation website Flight Global, said although a witness commented that the helicopter dropped “like a stone” there were indications the pilot might have still had “some ­ability to fly” before the impact.

He said: “This type of helicopter is sophisticated and robust. I think what has happened here is that you have an aircraft that became either uncontrollable or partially controllable. We just don’t know how much ­control the pilot did have in the final seconds of the flight. Something ­dramatic has suddenly occurred, probably mechanical failure of some kind.

“This one was a twin-engined aircraft. If you lose one engine you can fly on the other one and if you lose both engines then you can glide.

The police helicopter smashed into the Clutha pub on Friday night [PA]

“If we listen carefully to what the witnesses have been saying it doesn’t sound as if this helicopter was, at the end, able to glide.

“It may be that in the final moments the pilot had some control or was in some way able to reduce the rate of descent… although I suspect it was dropping pretty fast, dangerously fast for those on board.”

Mr Learmount said he thought that if the helicopter had been completely uncontrollable the crash could have been “even worse”.

The helicopter’s “black box” data recorder will be key in determining the cause of the crash and Mr Johnston said the Air Accident Investigation Branch will try to find it as soon as possible.